In a functioning democracy a majority of people can change the leader after some interval of time. If that majority is stupid or malevolent, well, you're fucked.
Someone did do something about bad leaders, as soon as someone with the balls appeared. Reinhard usurped the throne. Reinhard acknowledged that it was the duty of his subjects to rebel against his dynasty if the they thought they could lead better, whereas Yang took incompetent orders because he was too much of a coward to take responsibility for the galaxy's destiny.